Bitumen removal from tailings centrifuge centrate

ABSTRACT

A process for removing residual bitumen from oil sands tailings is provided, comprising optionally diluting the tailings with sufficient water to yield a tailings feed having a solids content in the range of about 18 wt % to about 36 wt %; adding one or both of a coagulant and a flocculant to the tailings feed to form a centrifuge feed; centrifuging the centrifuge feed to produce a cake and a centrate having a solids content of less than about 3 wt %; introducing the centrate into a flotation device so that bitumen froth and cleaned centrate are formed; recycling the cleaned centrate as dilution water or discharging the cleaned centrate to a tailings pond, and further processing the bitumen froth from the flotation device either by returning it to the primary Extraction feed or by treating it in a new or existing froth treatment plant.

FIELD OF THE INVENTION

The present invention relates to a process for removing residual bitumen from oil sands tailings, particularly from centrate derived from centrifugation of the tailings.

BACKGROUND OF THE INVENTION

Oil sand generally comprises water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules which contain a significant amount of sulfur, nitrogen and oxygen, The extraction of bitumen from oil sand using hot water processes yields large volumes of fine tailings composed of fine silts, clays, residual bitumen and water. Mineral fractions with a particle diameter less than 44 microns are referred to as “fines.”

When oil sand tailings are pumped to the deposition area, the coarse sands settle quickly on the beach while the fine tailings run off the beach and flow by gravity to the tailings ponds. The low density run-off material is referred to as thin fine tailings. The thin fine tailings suspension is typically 85% water and 15% fine particles by weight. Dewatering of fine tailings occurs slowly. After a few years when the fine tailings have reached a solids content of about 30-35% and are commonly referred to as fluid fine tailings (FFT) which typically contains about 2 wt % bitumen.

Attempts to recover bitumen from the FFT have been largely unsuccessful. Skimming bitumen from the tailings pond is no longer practiced due to high operating costs and difficulties in dealing with the froth produced. FFT may be processed for dewatering and reclamation through decanter centrifuges, producing a cake and a centrate (i.e., liquid stream). The centrate is usually recycled as dilution water or discharged to the tailings pond. However, centrate can have variable bitumen contents ranging from 0.01% to 1.9%. Higher centrate bitumen contents often result in slugs of bitumen discharged from the centrifuges. These high bitumen contents in centrate may cause problems for water systems during recycle, bitumen accumulations in the centrifuge feed that interfere with flocculation or cause mats of bitumen to form on the surface of tailings ponds.

Accordingly, there is a need for an improved method to remove residual bitumen from oil sands tailings centrifuge centrate.

SUMMARY OF THE INVENTION

The current application is directed to a process for removing residual bitumen from oil sands tailings, particularly from centrate derived from centrifugation of the tailings. The present invention is particularly useful with, but not limited to, fluid fine tailings. It was surprisingly discovered that by conducting the process of the present invention, one or more of the following benefits may be realized:

(1) the centrate represents an easy stream to remove bitumen due to having a relatively low solids content;

(2) the cleaned centrate may be recycled as centrifuge dilution water, without build-up of bitumen mats or slugs which can accumulate with time or in upset conditions; and

(3) if returned to the tailings pond, the cleaned centrate results in less bitumen being deposited upon the surface of the pond.

(4) the recovered bitumen has production value when returned to the oil sand processing plant.

Thus, use of the present invention provides both environmental and economic incentives for removing bitumen from the centrate derived from centrifugation of oil sands tailings.

In one aspect, a process for removing residual bitumen from oil sands tailings is provided, comprising:

-   -   optionally diluting the tailings with sufficient water to yield         a tailings feed having a solids content in the range of about 18         wt % to about 36 wt %;     -   adding one or both of a coagulant and a flocculant to the         tailings feed to form a centrifuge feed;     -   centrifuging the centrifuge feed to produce a cake and a         centrate having a solids content of less than about 3 wt %;     -   introducing the centrate into a flotation device so that bitumen         froth and cleaned centrate are formed;     -   recycling the cleaned centrate as dilution water or discharging         the cleaned centrate to a tailings pond; and     -   further processing the bitumen froth from the flotation device.         The bitumen froth may be processed either by adding it to the         primary extraction feed, or by treating it in a new or existing         froth treatment plant. In either case, the flotation bitumen         froth may be first pre-cleaned by gravity removal of a portion         of the free water phase.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings wherein like reference numerals indicate similar parts throughout the several views, several aspects of the present invention are illustrated by way of example, and not by way of limitation, in detail in the figures, wherein:

FIG. 1 is a schematic of one embodiment of the present invention for removing bitumen from centrate produced from the centrifugation of oil sands tailings.

FIG. 2 is a microscopic image of centrate froth showing the water continuous phase at the top (dark area) and the bitumen continuous phase at the bottom (fluorescence mode, 450-490 nm incident light).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments contemplated by the inventor. The detailed description includes specific details for the purpose of providing a comprehensive understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.

The present invention relates generally to a process for removing residual bitumen from oil sands tailings, particularly from centrate derived from centrifugation of the tailings. As used herein, the term “tailings” means tailings derived from oil sands extraction operations and containing a fines fraction. The term is meant to include fluid fine tailings (FFT) from tailings ponds and fine tailings from ongoing extraction operations (for example, thickener underflow or froth treatment tailings) which may bypass a tailings pond.

FIG. 1 is a flow diagram of the process of the present invention. In one embodiment, the tailings are primarily FFT obtained from tailings ponds. However, it should be understood that the fine tailings treated according the process of the present invention are not necessarily obtained from a tailings pond, and may also be obtained from ongoing oil sands extraction operations.

In a tailings pond, the tailings stream separates into an upper water layer, a middle FFT layer, and a bottom layer of settled solids. The FFT layer may be removed from between the water layer and solids layer via a dredge or floating barge having a submersible pump. The FFT may then be transported to a centrifuge plant for processing.

The FFT 10 is optionally diluted with water within a suitable vessel such as a tank. The source of water is preferably tailings water. Seepage of tailings water may arise from sand dike construction rather than from the pond, and can be stored in a water tank for use in processing. Sufficient water is added to achieve a centrifuge feed having a solids content in the range of about 18 wt % to about 36 wt %, preferably greater than about 30 wt %, for the most economic usage of the centrifuge equipment. Dilution provides a consistent feed to the centrifuge to ensure stable machine operation. Optionally, a coagulant is introduced into the in-line flow of FFT prior to entering a mixer. As used herein, the term “coagulant” refers to a reagent which neutralizes repulsive electrical charges surrounding particles to destabilize suspended solids and to cause the solids to coagulate. Suitable coagulants include, but are not limited to, gypsum, lime, alum, cationic polymers, or any combination thereof. In one embodiment, the coagulant comprises gypsum or lime. As used herein, the term “in-line flow” means a flow contained within a continuous fluid transportation line such as a pipe or another fluid transport structure which preferably has an enclosed tubular construction. In one embodiment, the dosage of the coagulant ranges from about 300 grams to about 1,500 grams per Tonne of solids in the FFT.

The FFT is then pumped from the tank into a mixer. Additional water and a flocculant are introduced into the in-line flow of the FFT at a line prior to entering the mixer. The source of water is preferably tailings water. As used herein, the term “flocculant” refers to a reagent which reacts with the FFT solids to form flocs and through rearrangement reactions increases the strength of the flocculated FFT. Flocculants useful in the present invention are generally anionic, nonionic, cationic or amphoteric polymers, which may be naturally occurring or synthetic, having relatively high molecular weights. Preferably, the polymeric flocculants are characterized by molecular weights ranging between about 1,000 kDa to about 50,000 kDa. Suitable natural polymeric flocculants may be polysaccharides such as dextrin, starch or guar gum. Suitable synthetic polymeric flocculants include, but are not limited to, charged or uncharged polyacrylamides, for example, a high molecular weight polyacrylamide-sodium polyacrylate co-polymer having a medium charge density (about 20-35% anionicity).

Other useful polymeric flocculants can be made by the polymerization of (meth)acrylamide, N-vinyl pyrrolidone, N-vinyl formamide, N,N dimethylacrylamide, N-vinyl acetamide, N-vinylpyridine, N-vinylimidazole, isopropyl acrylamide and polyethylene glycol methacrylate, and one or more anionic monomer(s) such as acrylic acid, methacrylic acid, 2-acrylamido-2-methylpropane sulphonic acid (ATBS) and salts thereof, or one or more cationic monomer(s) such as dimethylaminoethyl acrylate (ADAME), dimethylaminoethyl methacrylate (MADAME), dimethydiallylammonium chloride (DADMAC), acrylamido propyltrimethyl ammonium chloride (APTAC) and/or methacrylamido propyltrimethyl ammonium chloride (MAPTAC). The preferred flocculant may be selected according to the FFT composition and process conditions.

The flocculant may be supplied from a flocculant make up system for preparing, hydrating and dosing of the flocculant. Flocculant make-up systems are well known in the art, and typically include a mixing skid, one or more storage tanks, and a dosing pump. The dosage of flocculant is controlled by a metering pump. In one embodiment, the dosage of flocculant ranges from about 400 grams to about 1,500 grams per tonne of solids in the FFT. In one embodiment, the flocculant is in the form of a 0.2-0.4% solution.

The additional water is provided to disperse the flocculant into the forward flow of the FFT for better flocculation. The FFT and diluted flocculant are further combined within the mixer. The flocculated FFT is mixed in a manner so as to avoid overshearing which results in floc breakage and re-suspension of the fines within the water. Suitable mixers include, but are not limited to, simple pipe tee mixers, in-line static mixers, dynamic mixers, and continuous stirred-tank reactors (CSTR's). Preferably, the mixer is a tee mixer positioned before the feed tube of the centrifuge. Alternatively, the diluted flocculant may bypass the mixer and be fed directly through the feed tube of the centrifuge for addition to the FFT.

At a centrifuge plant 12, the flocculated FFT is transferred to a centrifuge for dewatering. In one embodiment, the centrifuge is a solid bowl decanter centrifuge. The cake 14 is collected and transported via a conveyor, pump or transport truck to a disposal area where the cake is stacked to maximize dewatering by natural processes.

In one embodiment, the centrate 16 has a solids content of less than about 3 wt %. The centrate 16 is transferred to a flotation device at a bitumen recovery plant 18 to recover as much bitumen from the centrate as possible in the form of froth 20. Bitumen recovery may be accomplished using various flotation devices including, but not limited to, a gravity separator, a mechanical flotation cell, a separator equipped with aeration downpipes, a flotation column, or any combination thereof.

In one embodiment, the centrate 16 may be transferred to a gravity separator to enable quiescent separation of the bitumen from the centrate 16. A gravity separator typically includes a shallow cone end and a rake at the bottom of the cone for further concentrating the bitumen froth by releasing any entrapped solids and water. Aeration of the centrate 16 promotes the attachment of bitumen to air bubbles, creating a lower-density bitumen froth 20 which floats to the upper portion of the gravity separator. The resulting bitumen froth 20 overflows the weir of the vessel into a launder extending around the rim of the gravity separator for removal for downstream processing.

In one embodiment, the centrate 16 may be transferred to a mechanical flotation cell which typically includes a mixer and diffuser mechanism at the bottom of the mixing tank to introduce air and provide mixing action, thereby intensifying the aeration of the bitumen droplets.

In one embodiment, the centrate 16 may be transferred to a separator equipped with aeration downpipes which combines the centrate 16 with air in a downpipe where high shear creates the turbulent conditions required for aeration of the bitumen droplets. The very high interfacial surface area and intense mixing result in rapid bitumen attachment to the air bubbles.

In one embodiment, the centrate 16 may be transferred to a flotation column which includes air spargers to introduce air at the bottom of a tall column while introducing the centrate above. The countercurrent motion of the centrate 16 flowing down and the air flowing up provides mixing action to aerate the bitumen droplets. Counter current froth washing with water may also be employed to produce an enhanced froth quality.

As an optional enhancement to any of the bitumen flotation embodiments, the flotation overflow may be cleaned by removal of a significant portion of the free water phase, using a gravity-based froth cleaner. This is done to reduce the impact of water and solids from the free water phase on downstream processing. The froth cleaner is usually a simple gravity separator, fed by the flotation overflow stream, and producing a cleaned froth overflow stream, and a water phase underflow stream, containing only small amounts of bitumen, to be returned to tailings for disposal. In one embodiment, the cleaned froth 20 contains 30-40% bitumen.

Following use of any one of the above flotation devices, froth cleaners or the like, the bitumen froth 20 is withdrawn and transferred to either the primary extraction feed, or directly to a froth treatment plant 22. The bitumen which is present in the bitumen froth 20 comprises both non-asphaltenic material and asphaltenes. Froth treatment is the process of eliminating the aqueous and solid contaminants from the bitumen froth 20 to produce a clean bitumen product for downstream upgrading processes. The bitumen froth 20 is diluted with a hydrocarbon solvent to reduce the viscosity and density of the oil phase, thereby accelerating the settling of any dispersed phase impurities by gravity or centrifugation.

Either a paraffinic or naphthenic type diluent may be used. Examples of paraffinic type diluents include C4 to C8 aliphatic compounds and natural gas condensate, which typically contains short-chained aliphatic compounds and may also contain small amounts of aromatic compounds. Examples of naphthenic type diluents include toluene (a light aromatic compound) and naphtha, which may be comprised of both aromatic and non-aromatic compounds. The difference in the bitumen produced by use of either a paraffinic or naphthenic type diluent can be attributed largely to the presence of aromatics. Aromatics have the ability to hold asphaltenes in solution, whereas paraffinic type diluents cause asphaltene precipitation.

Recovery of the hydrocarbon solvent from the diluted bitumen component is typically conducted in a recovery unit before the bitumen is delivered to a refinery for further processing.

The remaining bitumen-depleted, cleaned centrate 24 may be either recycled as dilution water in the centrifugation process or discharged back to the tailings pond 26.

Exemplary embodiments of the present invention are described in the following Examples, which are set forth to aid in the understanding of the invention, and should not be construed to limit in any way the scope of the invention as defined in the claims which follow thereafter.

EXAMPLE 1

In a first pilot test, centrate produced from the centrifugation of fluid fine tailings was collected in open topped rectangular tanks prior to discharge to a settling basin. The tanks served as gravity separators, and the froth formed on top of the centrate within the tanks. Samples of the froth were collected and analyzed. The froth was found to contain 57% bitumen and 9% solids. Without bitumen recovery, discharge of the centrate into the Mildred Lake Settling Basin resulted in the formation of a large visible mat of bitumen.

EXAMPLE 2

In a second pilot test, six samples of froth which formed in the centrate tank of an Andritz A14 centrifuge were taken. The analyses of the froth samples are set out in Table 1. The froth samples had considerably poorer froth quality compared to the first centrifuge pilot. The present FFT centrifuge prototype samples had bitumen contents ranging from 8.5 wt % to 28.4 wt %, with solids contents from 11.4 wt % up to 18.9 wt %.

TABLE 1 Date Sep. 28, Sep. 28, Sep. 28, Sep. 28, Sep. 29, Sep. 29, 2011 2011 2011 2011 2011 2011 Time 10:23 13:40 15:28 16:55 14:57 15:49 Bitumen, wt % 11.7 17.0 15.8 8.5 22.7 28.4 Water, wt % 68.6 66.9 65.0 72.2 60.9 56.3 Solids, wt % 16.8 15.8 18.9 18.3 11.4 12.4 Solids <5.5 55.1 60.9 62.7 54.6 55.9 56.2 μm, % Solids <44 96.5 97.8 98.3 96.9 96.3 96.4 μm, % Solids d50, 4.6 3.8 3.6 4.7 4.5 4.5 μmm

As shown in FIG. 2, the froth samples were mostly water-continuous. The bitumen continuous phase was found to have 30 vol % of degraded bitumen, with mostly dendrites and sheets ranging from 100 μm to 150 μm. The bitumen continuous phase was also found to contain a high amount of dispersed water droplets that were less than 10 μm in size. Both water continuous and bitumen continuous phases were observed, with the dispersed water drops in the bitumen continuous phase showing up as dark circles.

Further characterization tests were conducted on the bitumen from the centrate froth samples (Table 2). The bitumen was separated from the centrate froth samples using a combination of Dean Stark extraction with toluene, followed by removal of the toluene with the standard rotary evaporator method. The bitumen asphaltenes and MCR characterization data fall within the range of normally expected values.

TABLE 2 Date Sep. 28, Sep. 28, Sep. 28, Sep. 28, Sep. 29, Sep. 29, 2011 2011 2011 2011 2011 2011 Time 10:23 13:40 15:28 16:55 14:57 15:49 Extraneous 0.3 0.3 0.2 0.2 0.5 0.5 Matter, wt % C5 Asphaltene 15.1 15.3 15.0 13.8 16.2 16.6 Content, wt % Micro-carbon 12.9 12.8 12.6 12.6 13.4 13.8 Residue, wt %

EXAMPLE 3

The centrate froth samples were also processed using the 1 G and Cold Spin tests, as detailed below, to evaluate the simulated froth treatment processability in inclined plate settler and centrifuges.

Summary of Cold Spin Test

The cold spin test was performed as follows:

-   -   Collect a froth sample in a 250 ml jar, and add naphtha to         achieve a naphtha to bitumen ratio (N/B) of 0.7;     -   Heat to 80° C., then mix for 10 minutes on a shaker table;     -   Centrifuge for 10 minutes at 2000 RPM; and     -   Take a 1 gram sample of hydrocarbon layer, and determine water         content with a Karl Fischer Titration.

Summary of a 1 G Test

The 1 G test was performed as follows:

-   -   Collect a froth sample in a 1 litre jar, and add naphtha to         achieve a 0.7 N/B;     -   Heat to 80° C., then mix for 20 minutes on a shaker table;     -   Further mix with a baffled tank/impeller mixer at 700 RPM for 10         minutes;     -   Place the jar in a 80° C. water bath, and allow to settle for 2         hours;     -   At 0, 1, 3, 5, 7, 10, 15, 20, 30, 60, 90, 120 minute marks,         collect 1 gram samples, and determine water contents with a Karl         Fischer Titration.         The 1-G tests of the froth samples showed no froth separation at         all, possibly due to the high water and emulsion content of the         centrate froth samples. In the Cold Spin tests, it was found         that an average of 1% water remaining in the diluted bitumen,         which falls within the range of 1% to 2% expected for normally         separating froths (Table 3). The water content in the diluted         bitumen samples ranged from 0.25% to 3%. The tests indicate that         the froth would likely be difficult to process in the inclined         plate settlers, but may be treatable with a centrifuge-based         froth treatment process with naphtha dilution. This behavior is         consistent with the 30% degraded bitumen observed in the         microscopy evaluations.

TABLE 3 Date Sep. 28, Sep. 28, Sep. 28, Sep. 28, Sep. 29, Sep. 29, 2011 2011 2011 2011 2011 2011 Time 10:23 13:40 15:28 16:55 14:57 15:49 Cold Spin 0.88 0.73 0.65 0.95 0.65 1.13 Test Actual Naphtha/ Bitumen Ratio Cold Spin 0.8 0.47 0.56 0.25 3.01 1.1 Water in Diluted Bitumen, wt %

EXAMPLE 4

During the second pilot test, nine 1 m³ totes of FFT centrifuge centrate were collected for evaluation in a test loop. The centrate was circulated through a flow loop through an aerator, and transferred to an open topped tank where froth was removed. Three tests were performed, one with “normal” centrate and two with “off-spec” centrate containing a higher amount of solids and bitumen. Samples were collected for analysis, and a complete material balance was performed and shown in Table 4. These results indicate that the froth qualities are very “lean” at less than 10% bitumen content, potentially indicating the need for froth cleaning (i.e. removal of the free water phase) prior to further processing. Recovery of centrate bitumen is expected to have its highest production potential during periods of “off-spec” FFT centrifuge operation, when the centrate contains more bitumen

TABLE 4 Mass balances of centrate bitumen flotation tests Bitumen Water Solids Bitumen Water Solids Test# Centrate Name Content % Content % Content % Recovery % Recovery % Recovery % CC-1 off-spec Feed 0.99 90.55 8.45 100.00% 100.00% 100.00% Froth 4.32 85.55 10.14 36.60% 7.95% 10.09% Free bitumen 13.02 76.81 10.17 0.41% 0.03% 0.04% Tailings 0.68 91.02 8.30 62.99% 92.02% 89.87% CC-2 off-spec Feed 1.02 91.44 7.54 100.00% 100.00% 100.00% Froth 4.94 84.96 10.11 43.82% 8.41% 12.14% Free bitumen 7.41 82.86 9.73 0.12% 0.02% 0.02% Tailings 0.63 92.09 7.28 56.06% 91.58% 87.84% CC-3 on-spec Feed 0.06 99.66 0.28 100.00% 100.00% 100.00% Froth 7.31 80.14 12.55 45.17% 0.30% 16.70% Free bitumen 5.16 90.64 4.20 0.47% 0.01% 0.08% Tailings 0.03 99.73 0.24 54.36% 99.69% 83.21%

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the full scope consistent with the claims, wherein reference to an element in the singular, such as by use of the article “a” or “an” is not intended to mean “one and only one” unless specifically so stated, but rather “one or more”. All structural and functional equivalents to the elements of the various embodiments described throughout the disclosure that are known or later come to be known to those of ordinary skill in the art are intended to be encompassed by the elements of the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. 

1. A process for removing residual bitumen from oil sands tailings comprising: a) optionally diluting the tailings with sufficient water to yield a tailings feed having a solids content in the range of about 18 wt % to about 36 wt %; b) adding one or both of a coagulant and a flocculant to the tailings feed to form a centrifuge feed; c) centrifuging the centrifuge feed to produce a cake and a centrate having a solids content of less than about 3 wt %; d) introducing the centrate into a flotation device so that bitumen froth and cleaned centrate are formed; and e) recycling the cleaned centrate as dilution water in step (a) or discharging the cleaned centrate to a tailings pond. f) further processing the bitumen froth from the flotation device.
 2. [Currently Amended] The process of claim 1, further comprising cleaning the bitumen froth in a froth cleaner to produce a cleaned froth overflow stream and a water phase underflow stream having reduced bitumen for disposal.
 3. The process of claim 1, wherein in step (a), the solids content is greater than about 30 wt %.
 4. The process of claim 1, wherein in step (b), the tailings feed and flocculant are combined within a mixer.
 5. The process of claim 4, wherein the flocculant is added in-line prior to entering the mixer.
 6. The process of claim 5, further comprising diluting the flocculant.
 7. The process of claim 6, wherein the dosage of flocculant ranges from about 400 grams to about 1,500 grams per tonne of solids in the tailings.
 8. The process of claim 7, wherein the flocculant is the form of a 0.2-0.4% solution.
 9. The process of claim 8, wherein the flocculant comprises a polyacrylamide anionic flocculant.
 10. The process of claim 1, wherein in step (b), the flocculant is fed directly to the centrifuge.
 11. The process of claim 1, wherein the centrifuge is a solid bowl decanter centrifuge.
 12. The process of claim 1, wherein after step (c), the cake is disposed in an area using a dry stacking mode of disposal.
 13. The process of claim 1, wherein the tailings comprise fluid fine tailings.
 14. The process of claim 1, wherein the flotation device comprises a separation vessel, a mechanical flotation cell, a separator equipped with aeration downpipes, or a flotation column.
 15. The process of claim 1, wherein the bitumen froth from the flotation device is further processed by mixing it with a primary extraction feed or by treating it in a new or existing froth treatment plant. 